X-ray image intensifier tube having a non-specular backing for the scintillator layer



Oc 14. 1969 -c. w. BATES. JR. ETAL 3,473,

X-RAY IMAGE ENSIFIER TUBE HAVING A NON- CULAR BACK FOR THE SCINTILLATORLAYE Filed Aug. 25, 1967 men VOLTAGE 2 4 X-RAY commcfi GENERATOR\.

,3 ANODE VIEWING v B E SCREEN- //1x RAY BEAM FIGI 5. l4 FOCUS ELECTRODEAUORNEY FIG.3 2|

l INV'ENTORS 8 b CLAYTON w. BA JR. 9,\\\e e 9 BY RONALD L. BEL I UnitedStates Patent O US. Cl. 313101 4 Claims ABSTRACT OF THE DISCLOSURE Inthe improvement of the present invention a nonspecular surface isprovided between the scintillator layer and its mechanical supportivestructure, which may be the X-ray transmissive portion of the envelopefor producing improved performance of the X-ray image intensifier tube.In one embodiment, the non-specular surface is formed by roughening thesurface of the supportive structure which faces the scintillator suchthat the surface has a characteristic roughness greater than thewavelength of the optical photons emitted by the scintillator such thatthe photons are not trapped in the scintillating layer but are reflectedfrom the roughened surface back through the scintillator material to thephoto cathode to improve the conversion efiiciency of the imageintensifier tube. In another embodiment, the non-specular backingsurface is provided by a light absorptive layer disposed between thescintillator and the supportive structure for absorbing photonsback-scattered from the scintillator. In this manner, the definition ofthe X-ray image intensifier tube is substantially improved because theback-scattered photons are not reflected back to the photo cathodeemitter tube to form an image which would be more diffused than theimage obtained from the optical photons which are forward scattered. Theroughened face of the supporting layer is conveniently made by formingthis part of aluminum and anodizing its surface to form the non-specularsurface. The anodized coating is preferably white to provide arelatively high albedo. The light absorptive layer, as employed forimproved definition may, for example, be formed by a layer of carbon.

DESCRIPTION OF THE PRIOR ART Heretofore, it has been proposed to vapordeposit an activated alkali metal halide scintillator layer directlyupon an aluminum supporting structure of an X-ray intensifier tube. Suchan X-ray intensifier tube is described and claimed in copending US.application 606,514 filed Dec. 27, 1966, Pat. No. 3,402,792 and assignedto the same assignee as the present invention. Such an X-ray pickupscreen has improved stopping probability for X-rays as compared withprior X-ray pickup screens which have employed particulatedscintillating material such as, for example, zinc sulphide. The improvedstopping probability for X-rays and thus, conversion efficiency, arisesdue to the improved absorptive powers of the alkali metal halides suchas cesium iodide and due to the fact that the vapor-deposited layer hasa higher density than obtainable by a settled layer of particulated Zincsulphide material. However, in this prior device the scintillator layerwas deposited directly upon a clean and polished substrate memberforming the X-ray window of the pickup tube. In such a case it is foundthat a substantial proportion of the emitted optical photons, generatedin the scintillator layer, tend to be trapped in the scintillator layerdue to internal reflection and, thus, these trapped photons do not passinto the photo-emitting layer to contribute to the electron image. Thus,the intensifier tube has less than optimum 3,473,066 Patented Oct. 14,1969 conversion efficiency. Also, it is found that the photons that areback-scattered from the scintillator layer may be reflected from thesubstrate back through the scintillator layer to the photo-emittinglayer tending to diffuse the converted X-ray image. Therefore, it isdesired to provide means for inhibiting the light-trapping effect in thescintillator for X-ray image-intensifier tubes where high conversionefiiciencies are desired and it is desired to absorb the back-scatteredoptical photons in X-ray image intensifier tubes where high definitionis desired.

SUMMARY OF THE PRESENT INVENTION The principal object of the presentinvention is the provision of an improved X-ray image intensifier tube.

One feature of the present invention is the provision, in an X-ray imageintensifier tube, of providing a non-specular surface between the X-raytransmissive supporting member and the scintillator layer for improvingthe performance of the X-ray image intensifier tube.

Another feature of the present invention is the same as the precedingfeature wherein the non-specular surface comprises a roughened surfacehaving a characteristic surface roughness greater than the wavelength ofthe optical photons emitted in the scintillator layer, whereby trappingof optical photons in the scintillator layer is inhibited to improve theconversion efficiency of the image intensifier tube.

Another feature of the present invention is the same as the precedingfeature wherein the non-specular surface is provided by an anodizedaluminum surface having a relatively high albedo, whereby the conversionefficiency of the tube is substantially improved.

Another feature of the present invention is the same as the firstfeature wherein the non-specular surface is provided by a layer oflight-absorptive material, whereby the definition of the converted X-rayimage is substantially improved.

Another feature of the present invention is the same as the precedingfeature wherein the layer of light-absorptive material may, for example,be formed by carbon.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic line diagram ofan X-ray image intensifier tube,

FIG. 2 is an enlarged cross-sectional view of a portion of the structureof FIG. 1 delineated by line 2-2 and depicting the prior art,

FIG. 3 is a view similar to that of FIG. 2 depicting a pickup screenconstruction of the present invention, and

FIG. 4 is a view similar to that of FIG. 3 depicting an alternativeconstruction of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 thereis shown an X-ray system employing an X-ray image intensifier tube 2,Such a system is described in an article entitled, X-ray ImageIntensification With a Large Diameter Image Intensifier Tube, appearingin the American Jnl. of Roentgenology Radium Therapy and NuclearMedicine, vol. 85, pp. 323-341 of February 1961. Briefly, an X-raygenerator 3 serves to produce and direct the beam of X-rays onto anobject 4 to be X-rayed. The image intensifier tube 2 is disposed toreceive the X-ray image of the object 4.

The image intensifier tube 2 includes: a dielectric vacuum envelope 5,as of glass, approximately 17 inches long and 10 inches in diameter. Thepickup face portion 6 of the tube 2 comprises a spherical X-raytransparent portion of the envelope 5, as of aluminum or conductiveglass, which is operated at cathode potential. An image pickup screen 7(see FIG. 2), made of an X-ray sensitive scintillator such as anactivated alkali metal halide, is deposited onto the inside sphericalsurface of the envelope portion 6 to a thickness as of 0.01". Achemically inert optically transparent buffer 8 is coated over thescintillator layer 7. A photo cathode layer 9 is formed over the bufferlayer 8.

In operation, the X-rays penetrate the object 4 to be observed. Thelocal X-ray attenuation depends on both the thickness and atomic numberof the elements forming the object under observation. Thus, theintensity pat tern in the X-ray beam after penetration of the object 4contains information concerning the structure of the object. The X-rayimage passes through the envelope section -6 and falls upon the X-raysensitive scintillator layer 7 wherein the X-ray photons are absorbedand re-emitted as optical photons. The optical photons pass through thetransparent buffer 8 to the photo cathode 9 wherein they produceelectrons. The electrons are emitted from the photo cathode in a patternor image corresponding to the original X-ray image. The electrons areaccelerated to a high velocity, as of 30 kv., within the tube 2 and arefocused through an anode structure 12 onto a fluorescent screen 13 forviewing by the eye or other suitable optical pickup device. Electronfocusing electrodes 14 are deposited on the interior surfaces of thetube to focus the electrons through the anode 12.

In the intensifier tube 2, one 50 kev. of X-ray energy absorbed by theX-ray sensitive scintillator screen 7 produces in the case of Na dopedCsI about 2,000 photons of blue light. These two thousand photons ofblue light produce about 400 electrons when absorbed in the photocathode layer 9. The 400 electrons emitted from the photo cathodeproduce about 400,000 photons of light in the visible band when absorbedby the fluorescent viewing screen 13. Thus, the X-ray image is convertedto the visible range and greatly intensified for viewing.

Referring now to FIG. 2 there is shown a prior art X-ray pickup screenin greater detail. In operation, an X-ray or gamma ray passes throughthe X-ray transmissive envelope face 6 and is absorbed in thescintillator layer 7 giving rise to emission of optical photons. Some ofthe emitted photons travel in the direction of the original X-ray orgamma ray. Other optical photons are emitted or are back scatteredtoward the face 6 of the X-ray image intensifier tube 2. It turns outthat for relatively dense scintillator layers of alkali metal halidematerial such as, for example, cesium iodide, that optical photons whichare not within a relatively small cone, indicated by the solid angle a,will be trapped in the scintillator layer 7 by being reflected from thesurfaces of the scintillator 7 due to the difference in dielectricconstant of the scintillator 7 and the dielectric constant of the layers6 and 8 on opposite sides of the scintillator 7. The angle a is referredto as the angle of internal reflection and for cesium iodide with a CsSb photocathode overlay this angle is approximately 62. Thus, arelatively large percentage of the photons emitted by the scintillator 7are trapped in the scintillator layer 7 and do not contribute to thephoton image to be picked up by the photo cathode layer 9 to produce theelectron image. This tends to detract from the conversion efliciency ofthe image intensifier 2.

Referring now to FIG. 3 there is shown an X-ray pickup screenconstruction embodying features of the present invention. Morespecifically, the surface of the X-ray transmissive window 6 of theimage intensifier tube 2 which faces the scintillator layer 7 isroughened to a characteristic surface roughness greater than thewavelength of the optical photons emitted in the scintillator layer 7.This roughened surface is indicated at 21. The roughened surface 21forms a non-specular backing for the scintillator layer 7 causing thephotons which are back-scattered to surface 21 to be reflected at angleshaving a higher probability of falling within the solid angle on, suchthat a relatively high percentage of the photons which are emitted fromthe scintillator layer 7 will pass out of the layer 7 to the photocathode 9 where they can contribute to the electron image. Thus, theconversion efliciency of the structure of FIG. 3 is substantiallyimproved as compared with the prior art structure, wherein the surfaceof the substrate member 6 which faces the scintillator layer 7 providesa specular, i.e. mirror-like reflection of optical photons incidentthereon.

The roughened surface 21, as previously mentioned, has a characteristicsurface roughness greater than the wavelength of the optical photonsemitted by the scintillator layer 7. This means that the surfaceroughness will generally be at least on the order of 1 micron andshould, of course, be less than the thickness of the scintillator layer7. Suitable alkali metal halide scintillator materials forming thescintillator layer 7 include activated, cesium iodide, potassium iodide,sodium iodide, rubidium iodide, cesium bromide, and lithium iodide.

In one preferred embodment of the present invention, the X-ray window 6is formed by a sheet of aluminum and the roughened surface 21 isprovided by anodizing the surface of the aluminum substrate member 6.Such an anodized surface will be white and will have a high albedofurther improving the conversion efliciency of the X-ray imageintensifier tube. In case the pickup face 6 is made of a conductiveglass, the glass may be roughened as by etching the surface facing thescintillator layer 7. Its reflectivity may also be improved by coatingthe face with a flash coating of aluminum and then anodizing thealuminum to produce a white, rough surface having the high albedo.

Referring now to FIG. 4 there is shown an alternative X-ray pickupscreen embodiment of the present invention. This screen construction issubstantially the same as that of FIG. 2 except that a light absorptivelayer 22 is provided between the scintillator layer 7 and the insidesurface of the X-ray transmissive window 6. X-rays or gamma rays passingthrough the window 6 are absorbed in the scintillator layer 7 to produceoptical photons which are emitted from the scintillator layer 7. Some ofthese photons are emitted within the angle a and pass out of thescintillator 7 through the buffer to the photo cathode layer 8 whereinthey are absorbed and cause emission of electrons to form the electronimage. Those photons which are back-scattered from the scintillatorlayer are absorbed by the light-absorptive layer 22 such that theoptical image emitted from the scintillator layer 7 comprises an imageof point sources, thereby providing improved definition of the convertedX-ray image. Suitable light absorptive layers 22 include dark coatingssuch as, for example, a thin carbon coating formed on the inside surfaceof the window 6. A suitable thickness for this layer 22 is 10 microns.

The embodiments of the present invention have been described with abuffer layer 8, as of magnesium oxide, aluminum oxide, or lithiumfluoride. Such buffer materials are evaporated in vacuum onto thescintillator layer 7 to a suitable thickness such as 10,000 A. andpreferably 1,000 A. less. Use of the buffer layer 8 is not a requirementand, in some instances, the photo cathode emitting layer 9 may bedeposited directly upon the scintillator layer 7 in a manner asdescribed and explained in copending US. patent application 606,513filed Dec. 27, 1966 and assigned to the same assignee as the presentinvention.

As used herein, X-ray is defined to mean photons of an energy levelequal to those of X-rays or higher. Thus, X-ray as used herein means notonly X-rays but gamma rays and other high energy photons.

Since many changes may be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In an X-ray image intensifier tube, means forming an evacuatedenvelope having an aluminum X-ray transmissive window portion, meansforming a scintillator layer of activated alkali metal halide materialdisposed inside said envelope and overlaying said X-ray window forconverting X-ray images into optical images, means forming a photocathode layer overlaying said scintillator layer for converting theoptical images into electron images, means forming an electronaccelerating and focusing electrode structure for accelerating theelectron images to relatively high energy, means for picking up the highenergy electron images for viewing, the improvement comprising, meansforming a non-specular anodized surface formed on said aluminum windowportion of said envelope, said scintillator layer being disposedabutting said non-specular surface of said window portion, and saidanodized surface having a characteristic surface roughness greater thanthe wavelength of the optical photons emitted from said scintillatorlayer to inhibit light traveling in said scintillator layer.

2. In an X-ray image intensifier tube, means forming an evacuatedenvelope having an X-ray transmissive window portion, means forming ascintillator layer of activated alkali metal halide material disposedinside said envelope and overlaying said X-ray window for convertingX-ray images into optical images, means forming a photo cathode layeroverlaying said scintillator layer for converting the optical imagesinto electron images, means forming an electron accelerating andfocusing electrode structure for accelerating the electron images torelatively high energy, means for picking up the high energy electronimages for viewing, the improvement comprising, means forming anon-specular light absorptive layer disposed intermediate said windowportion of said envelope and said scintillator layer for absorbing theoptical photons back-scattering from said scintillator layer to improvethe definition of the optical image.

3. The apparatus of claim 2 wherein said light absorptive layer is adark coating.

4. The apparatus of claim 3 wherein said light adsorptive layer is alayer of carbon.

References Cited UNITED STATES PATENTS 2,312,206 2/1943 Calbick 313-112X 2,680,205 6/1954 Burton 313-116 X 2,739,243 3/1956 Sheldon 313-101 X2,955,218 10/1960 Schmidt 313-116 X 3,243,626 3/1966 Helvy et al 313-116X JAMES W. LAWRENCE, Primary Examiner DAVID OREILLY, Assistant ExaminerUS. Cl. X.R.

